First determination of $CP$ violation parameters from K$^0$-$\bar{K}^0$ decay asymmetry

Abstract We report the first determination of CP violation parameters from parameter from particle—antiparticle asymmetry in the decay of neutral kaons into two charged pions. Observation of such an asymmetry is direct proof of CP violation. A fit to the asymmetry enabled a determination of the parameter η +− to be made, yielding the result | η +− |=[2.32±0.14 (stat.)±0.03 (syst.)]×10 −3 and σ +− =42.3°±4.4° (stat.)±0.4° (syst.), with an additional uncertainty of ±1.0° due to the error on the present published value of Δm , the K L 0 -K S 0 mass difference. The magnitudes of both statistical and systematic errors will be significantly reduced in the future.